532 
Abstract Report of Agricultural Discussions. 
revived under the new name of the vegetable mould theory ; and 
serious apprehensions are entertained by many intelligent, educated 
peoplo that, by degrees, we may relapse into a condition wherein it 
will be difficult to grow remunerative crops on the soils of Eng- 
land. Such apprehensions could not possibly have been enter- 
tained had correct views existed as to the way in which plants 
take xip their food. The old humus theory has been successfully 
annihilated ; indeed, after Liebig's forcible writings in exposing its 
fallacy, it is impossible for it to be maintained. Yet tip to this day, 
readers of respectable journals arc addressed in lengthy papers, on 
the so-called vegetable mould theory, which are anything but satis- 
factory to those acquainted with the recent progress of scientific 
research in relation to the process of vegetable nutrition. 
In dealing with the subject of the atmospheric nutrition of plants 
very little can be said that is original, or of very recent date ; for 
within the last five or six years no very striking discoveries bearing 
upon the subject have been made. Our knowledge, however, in the 
absence of any material additions, has become more definite and 
positive, so that we can speak with greater certainty on matters of 
which we had formerly but indistinct ideas. 
The atmosphere is a mechanical mixture, chiefly of two gases ; 
oxygen constituting about 21 parts in round numbers, and nitrogen 
79. Mixed with these two gases are carbonic acid, ammonia, and 
nitric acid in certain small proportions ; water is also invariably 
present in the shape of vapour. It is interesting to notice that 
nitrogen, the most conspicuous constituent of the atmosphere, docs 
not take any direct part in the nutrition of plants, but it is pre- 
cisely those small quantities of matters for a long time overlooked — 
viz., ammonia and nitric acid, that arc most active and influential in 
the process ; for notwithstanding some experiments recently made in 
France by M. Ville, it has been decisively shown by M. Boussingault, 
in France, and Messrs. Lawes and Gilbert, in England, that nitrogen, 
as an element of the atmosphere, is not assimilated. Oxygen, the 
other great constituent of the air, has an influence rather indirect 
than immediate in the nutrition of plants. Plants, through the me- 
dium of their leaves, restore oxygen to the atmosphere, and thereby- 
keep up the balance which it is so necessary to maintain, but which 
is continually disturbed by the removal of the oxygen by animals 
for the support of respiration. We have abundant proof that it 
is chiefly the carbonic acid of the atmosphere that supplies the 
carbon of plants, or, in other words, the great bulk of all vegetation. 
It is calculated, indeed, that at least three-fourths of the dry sub- 
stance of plants is derived from the carbonic acid of the atmosphere. 
During the daytime, the absorption takes placo continuously; and, 
no sooner have the leaves absorbed carbonic acid than they set about 
the work of destroying its form, assimilating the carbon, manufacturing 
it into starch, gum, sugar, and other combinations found in all vegetable 
productions, and at the samo time throwing off the oxygen so as to 
restore the balance. 
It is by porno supposed that the decomposition of carbonic acid 
